This invention relates to thermal printing apparatus and in particular to the construction of means to feed items past a thermal print head for printing impressions on the items. The feeding means is particularly intended for use in franking machines for feeding mail items past a thermal print head.
Recently franking machines have been proposed in which thermal print heads are utilised to print franking impressions on mail items. The thermal print heads include a plurality of thermal printing elements disposed in a row extending transversely of a direction of feed of the mail items. A thermal transfer ink ribbon comprising a backing film carrying a transfer layer of ink is fed between the thermal elements and the mail items with the backing film in contact with the thermal elements and the ink layer in contact with the surface of the mail items. Selective heating of the thermal elements as the mail item is fed past the elements is effective to transfer ink selectively to the mail item and to build up row by row a franking impression on the mail item. The thermal printing requires that the ink transfer ribbon is maintained in heat transfer relationship with the thermal elements and that the front surface of the mail item which is to receive the impression is maintained in ink transfer engagement with the ink layer. Accordingly in known constructions of franking machine utilising a thermal print head and a thermal transfer ink ribbon a roller is disposed opposite the thermal elements and applies pressure to a rear surface of the mail item to ensure that the required heat transfer relationship between the ribbon the thermal elements and the required ink transfer engagement between the mail item and the ink layer is maintained during printing. The roller is constructed to be resiliently deformable or is mounted in a pivoted cradle which permits the roller to move toward and away from the thermal elements so as to accommodate mail items of different thicknesses.